overlay/042: remove wrong check for empty index

overlay/042: remove wrong check for empty index

The check for empty index in this test was wrongly copied from test
overlay/034. In test overlay/034 lower file starts as a hardlink, so
nlink accounting is done from the first copy up and index can be cleaned
on last upper hardlink unlink. In this test, lower starts as
non-hardlink, so first copy up does not perform nlink accounting and
therefore, union nlink count does not drop to 0 at the end of the test
and the index is not expected to be cleaned.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>